Known as a method of judging the truth of a bill is a method of judging the truth of a bill to be examined by comparing detection data obtained by a single sensor such as a magnetic sensor from the bill to be examined with reference data previously produced (JP-B-60-215293).
In this method, however, only the data detected by the single sensor is used for judging the truth. If it is judged how sensor is used as the sensor, it is easy to fabricate such a false bill that data which is judged to be a true bill by the sensor is obtained. That is, it is easy to counterfeit a bill.
Therefore, a method of judging the truth of a bill using two types of sensors (see JP-A-51-90890 and JP-A-51-90891). That is, a first sensor for detecting the transmission rate of visible light in a portion to be examined of a bill to be examined and a second sensor for detecting the transmission rate of infrared light in the portion to be examined of the bill to be examined are prepared, and it is judged whether or not the ratio of the detection level of the transmission rate of the visible light detected by the first sensor to the detection level of the transmission rate of the infrared light detected by the second sensor or the difference therebetween is within a predetermined range, to judge the truth of the bill to be examined.
Although this method also uses the two sensors, however, the truth is judged by simple judgment whether or not the difference or the ratio between the detection levels of the sensors is within a predetermined range. If it is judged how sensors are used as the sensors, it is easy to fabricate such a false bill that data which is judged to be a true bill by the sensors is obtained. That is, it is easy to counterfeit a bill.
An object of the present invention is to provide a method of judging the truth of a paper type which is more difficult to counterfeit.